Reactivation of aerobic granular sludge for the treatment of industrial shipboard slop wastewater: Effects of long-term storage on granules structure, biofilm activity and microbial community

This work reports on reactivation of aerobic granular sludge (AGS) for the treatment of industrial recalcitrant wastewater (slop wastewater) characterized by high salinity and hydrocarbons. AGS were reactivated in two reactors, R1 and R2, to treat industrial slop wastewater after a long-term storage for 12-months at 4 °C. In R1, salt-adapted mature aerobic granules were previously subjected to a step-wise increase of hydrocarbons, whereas in R2 aerobic granules were previously cultivated in presence of salinity and hydrocarbons. After a short-term reactivation period, the slop dosage caused a simultaneous decrease of granules dimensions and proteins/polysaccharides (PN/PS) ratio down to 1.76 (R1) and 2.35 (R2). Then, the increase of granules dimensions and of the PN/PS ratio up to 5.63 (R1) and 4.22 (R2), suggested a new granulation process. The difference in total petroleum hydrocarbons (TPHs) removal efficiencies (≈87 % R1, and ≈97 % R2) was mainly due to a biodegradation effect in R2 where granules were previously cultivated with slop. This was confirmed by a simultaneous decrease of TPHs concentration in the bulk and adsorbed into granules. Based on Next Generation Sequencing of metagenomic 16S encoding genes, in R2 typical oil-polluted seawater-associated bacteria were detected, which probably played key roles in TPHs biodegradation.